Pre-*** contains very little amount of semen and is next to impossible to impregnate someone. How old are you, and how long have you had your period? It can take years for your period to become regular. Also, there are many factors that can change your period such as, stress, change in eating habits, and exercise. It could very well be a side affect of the birth control, which could also explain your other symptoms. Birth control basically tricks your body into thinking that your pregnant by stopping your body from ovulating. So it could expliain the pregnancy like symptoms.
i dont know if its gunna help u but iv been on the implant since november. my periods are completely messed up, i have severe pains, dizzyness etc. i thought i lost a baby not long after i had my implant in but i will never know as i cant get to hosp. my guess is that what your feeling is because of stress or anxiety maybe? if and when i have a period, i can have 2 week long ones or 2 in a month. hope iv helped, lemme no if theres anything else